Zoom projection lens

ABSTRACT

A projection lens system primarily for a motion picture projector is a glass and plastic lens design enabling lower manufacturing cost than a comparable all glass design of similar optical performance. In the preferred embodiment, each of the plastic elements is formed with an aspheric surface.

nited Stal Linke Nov. 18, 1975 ZOOM PROJECTION LENS 3.194.139 7/1965Babcock 350/184X [75] Inventor: Walter R. Linke, Chicago, Ill. [73]Assignee: Bell & Howell Company, Chicago, Primary Examinerp aul sachet mAttorney, Agent, or Fzrm-J0hn E. Peele, Jr.

[22] Filed: Oct. 15, 1974 [21] App]. No.: 514,340 [57 ABSTRACT 52 us. 01350/184; 350/175 NG; 350/189; A pmjecfim P System g f f f a "A 350/216tlgie prlojector 1s glase an pastlc ens 651g}; enli 511 Im. c1. G02C15/00; 0020 3/04 a mg 3 P. a Compara e a 58 Field of Search 350/184, 175NO, 189, 216 glass desgn ferred embodlment, each of the plastlc elements15 [56] References Cited formed with an aspheric surface.

UNITED STATES PATENTS 1 Claim 17 Drawing Figures 3,000,260 9/1961T111016 e161. 350/184 l L L3 2 -4 L FILM TELEPHOTO R x 14, CONDITION R 7D 9 w I DI D2 s2 D3 I s g 5 WIDE ANGLE 1 CONDITION 3 IO US. Patent Nov.18, 1975 Sheet10f3 3,920,315

2 3 L4 FILM PLANE TELEPHOTO R CONDITION WIDE ANGLE CONDITION ZOOMPROJECTION LENS This invention relates to a projection lens system formotion picture projectors. More particularly, the lens system relates toa glass and plastic lens design enabling a lower manufacturing cost thancomparable all glass lens designs of similar performance characteristicsand optical corrections.

In recent years, zoom projection lenses have enjoyed a great deal ofpopularity with amateur motion picture enthusiasts. This popularity isenhanced by the ability of the lens to permit adjusting a projectedimage size while leaving the projector at a convenient location. Thecapability-of adjusting the lenses between relative telephoto towide-angle focal lengths overcomes the price differential usuallypresent in that the all glass zoom lens designs are generally moreexpensive to manufacture than comparable single focal length projectionlens designs of similar optical correction.

Manufacturing of glass elements requires an amount of grinding andpolishing of each element resulting in a significant cost beingattributed to each lens. To reduce the cost of the lens, an expedient isto replace selected glass elements with plastic elements, which can bemolded to shape. Since plastic elements can be mass produced by moldingat substantially less manufacturing costs than by grinding and polishingof glass and the raw plastic is of lower costs than glass, the finalcost of a lens with plastic elements is lower than comparable lens withall glass elements. Generally, in the past, the use of plastic elementsrequired more elements and resulted in lowering of selectedcharacteristics and optical performance of the lens system. However, bymolding one or more plastic elements with aspheric surfaces at little,if any, increase in cost of design and manufac- 2 lens system shown inFIG. 1, and having the design data given in Table l; and

FIGS. 3a to 311 are graphical representations of the various aberrationsfor the wide angle condition of the lens system of FIG. 1.

With reference to FIG. 1 of the drawings, the preferred variable focallength projection lens system comprises four components as I to IV withfive elements L to L The first component I is a positive meniscus typelens element L, with the outer surface S of a spherical section and theinternal surface S, of an aspheric section. By forming the asphericsurface on the element, correction by the element serves to correctdistortions, such as coma, in the system to a greater degree than woulda true glass meniscus element. The element L, is made as by molding ofplastic material, such as polymethylmethacrylate, and results in a lightweight, highly corrected element which can be more economicallymanufactured than by grinding and polishing of a similar glass element.

The second component II consists of a negative element L moldedpreferably of the same plastic material. This element, air spaced fromelement L has an aspheric surface 5;, proximate element L and a slightlynegative opposite surface S... This element L, while providingcorrections for other optical aberrations tends to define the back focallength of the lens system.

Component III comprises element L a positive element molded of plasticmaterial with the surface S adjacent element L of spherical form and thesurface S ture, the characteristics and corrections can be made tomatch, if not exceed these corrections of a comparable all glass lensdesign.

An object of the invention is to provide a highly corrected zoomprojection lens system with a combination of plastic and glass elements,at least one of the plastic elements having an aspheric surface.

The above and other objects of the invention will be apparent byreference to the detailed description when taken in conjunction with theaccompanying drawings, wherein like numerals refer to like parts.

In the drawings:

FIG. 1 is a diagrammatic view of a preferred formof the optical systemof the invention in relatively long focal length or telephotoorientation and relatively short focal length or wide angle orientation;

FIGS. 2a to 2h are graphical representations of the various aberrationsfor the telephoto condition of the remote therefrom of aspheric form.Element L is air spaced from elements L and component IV.

Component IV comprises elements L and L Element L is slightly positiveelement manufactured of glass material with both surfaces S S ofspherical form. Element L is relatively thick negative elementmanufactured of glass material. This element L is closely spaced withrespect to element L As shown in the upper and lower portions of FIG. 1,certain components of the lens system are movable relative to othercomponents to alter the focal length of the system. In the upperportion, showing the longer focal length arrangement or telephotocondition, components I and III are displaced axially forward fromcomponents II and IV. The shorter focal length arrangement or wide anglecondition is shown in the lower portion with the components I and IIIcloser to respective components II and IV. Typically, the lens design asset forth in this preferred example is capable of effective focallengths variable between approximately 20 and 30 millimeters.

The optical values of the optical system of Fig. 1 are as follows:

TABLE 1 TABLE l-continued THICKNESS LENS RADll (IN.) (IN) SPACINGS (IN.)

S, .5521 BFL In the above table, the first column lists the lenselements numerically, starting at the ray exit side of the system. Eachof the plastic elements, L to L has the same dispersive index andrefractive index, i.e. V=57.4 and N =1.49l7. The glass element L has adispersive index (V) of 58.6 and a refractive index (N of 1.613. Theelement L is of glass with a dispersive index (V) of 26.1 and arefractive index (N of 1.785. The second column lists the respectiveradii and the vertex radii of the aspheric surfaces *R *R and *R andwherein said aspheric surfaces are obtained from the formula;

where UK is the vertex radius of curvature of the surface of theelement; H is the Cartesian coordinate of a point-orf the surfacereferred in a vertical axis; Z is the Cartesian coordinate of a point onthe surface referred in a horizontal axis. The third column lists thethickness of the respective elements. The fourth column lists the axialspacings between the respective elements and the nominal image plane.Respectively, the effective focal length of the elements are 4.1971,-l.3567, 1.4462, 0.6287 and 1.0412 in inches.

FIG. 2a to 2h graphicallyrepresent various aberrations related to thetelephoto condition of this preferred form of the lens system, as shownin FIG. 1 and having the design data recited in Table 1. FIG. 2arepresents axial chromatic correction of three typical wavelengthsforming rays on axis and lateral.

FIG. 2b represents off axis aberrations of a ray passing from the zoneof the film format and through the lens transversely and tangentially.FIG. represents the aberrations of the rays passing from the comer ofthe film format through the lens tangentially and transversely. FIG. 2drepresents the radial or longitudinal aberration from the zone of thefilm format of rays entering the lens at 3 oclock, while FIG. 2erepresents similar aberrations from full field or corner rays. FIG. 2frepresents distortion as a percentage of a perfect image. FIG. 2grepresents the spherical aberration by a full line and theoffense-against-sine-condition by the dotted line. FIG. 2R representsthe curvature of field with tangential curvature being shown in fullline and sagittal curvature being shown in dashed line.

FIGS. 30 through 3R represent similar aberrations related to the wideangle condition of the lens system as those above identified withrespect to the telephoto condition.

What is claimed is:

1. An optical system of a highly corrected zoom projection lensincluding a combination of glass and plastic elements, at least one ofthe plastic elements having an aspheric surface, the system havingsubstantially the following specifications:

TABLE 1 THICKNESS LENS RAD (IN.) (IN.) SPACINGS (INJ 5 .1500 I0 .5900 L,R, 2.4600

.soon m .1500 L R 0.6410 1).; .2020

- S .0007 L; R; 0.8390 D .3365

S .5521 BFL wherein the first column lists the lens element numericallystarting at the ray exit side of the system, the plastic elements havingthe same dispersive index and refractive index i.e., V=57.4 andN,,=l.4917, glass element L; having a dispersive index V of 58.6 and arefractive index N of 1.613, the glass element L having a dispersiveindex V of 26.1 and a refractive N of 1.785; the second column lists therespective radii and vertex radii of the aspheric surfaces *R *FQ, and*R,, the negative values of the radii indicate surfaces which areconcave; the third column lists the thickness D, to D of the respectiveelements; and the fourth column lists the axial spacings S to S betweenthe respective elements and the image plane.

1. An optical system of a highly corrected zoom projection lensincluding a combination of glass and plastic elements, at least one ofthe plastic elements having an aspheric surface, the system havingsubstantially the following specifications: